Petroleum products generally exhibit a large variation in viscosity with variation in temperature. For lubricating oils used for automobiles or the like, it is preferable that such a temperature dependence of viscosity is small. In order to decrease the temperature dependence of viscosity, an ethylene/α-olefin copolymer having an effect of improving viscosity index is widely used as an ingredient blended in lubricating oils.
Further, lubricating oils lose fluidity at low temperatures because wax components therein solidify to crystals. In order to lower the solidifying temperature, lubricating oils also contain a pour-point depressant. The pour-point depressant prevents a three-dimensional network structure from forming through crystallization of wax components in lubricating oils, lowering the pour point of the lubricating oils.
Among low-temperature characteristics of lubricating oils which contain an ethylene/α-olefin copolymer and a pour-point depressant, the viscosity at a high shear rate is determined by compatibility between a lubricating oil base and the ethylene/α-olefin copolymer, while the viscosity at a low shear rate is strongly affected by the pour-point depressant. It is known that when an ethylene/α-olefin copolymer having a specific composition is used, the effect of a pour-point depressant is remarkably lowered through interaction of the copolymer with the pour-point depressant (refer to Patent document 1 and Patent document 2).
For this reason, an ethylene/α-olefin copolymer blended in lubricating oils, especially lubricating oils which are requested to have excellent low-temperature properties, is required not to impair the function of a pour-point depressant as well as to possess excellent performance of improving viscosity index.
In order to avoid the interaction between a pour-point depressant and an ethylene/α-olefin copolymer, it has been proposed to use, as a viscosity index improver, an ethylene/α-olefin copolymer having ununiform composition distribution, which is obtained by using a specific polymerization apparatus under specific polymerization conditions (refer to Patent document 3). However, no lubricating oils excellent in low-temperature characteristics at any shear rate were obtained.
As a method for improving low-temperature characteristics of lubricating oils, there is also mentioned a method where an ethylene/propylene copolymer having a high ethylene content is added as a viscosity index improver. In this method, low-temperature characteristics were improved with increasing ethylene content, but in some cases the ethylene sequence in the viscosity index improver crystallized at low temperatures, lubricating oil compositions themselves jellified, lowering handleability.
Further, even in a case where a copolymer having a high ethylene content was suitably used as a viscosity modifier for lubricating oils, the composition sometimes jellified at low temperatures when the molecular weight was over a given range or the ethylene copolymer had a wider composition distribution even a little or the like
Thus, property tolerance of the employed copolymer can not be so wide, and the properties need to be strictly controlled.
In the above circumstances, the present inventors have intensively investigated and found that a specific copolymer which comprises a structural unit derived from ethylene, a structural unit derived from an α-olefin having 3 to 19 carbon atoms, and a structural unit derived from an α-olefin having 4 to 20 carbon atoms does not impair the function of a pour-point depressant due to the above-mentioned interaction and that the amount of copolymer blended can be reduced because a high molecular weight copolymer can be prepared. That is, the copolymer works as a viscosity modifier for lubricating oils which is excellent in oil-thickening ability. Further, this copolymer has suitably adjusted compatibility with a lubricating oil base at low temperatures, providing lubricating oil compositions excellent in low-temperature characteristics in entire range of shear rate and excellent in handleability at low temperatures. Based on this finding, the present invention has been accomplished.
Patent document 1: U.S. Pat. No. 3,697,429;
Patent document 2: U.S. Pat. No. 3,551,336;
Patent document 3: Japanese Patent Laid-Open Publication No. 228600/1985.